The present invention relates to the use of closed loop mixed refrigerants in cryogenic separation of hydrocarbons and associated gaseous components.
U.S. Pat. No. 2,041,725 describes a single pressure, multistage, mixed refrigerant system. This early process appears to have served as a model for many later prior art, multistage mixed refrigerant systems. A practice has developed in multicomponent, cascaded refrigerant systems to have sequentially colder partial condensation stages in which the mixed refrigerant is partially condensed and separated at high pressure and then remixed at low pressure. In addition to imposing a significant thermodynamic inefficiency on the system by such separation and re-mixing, proper component distribution in the several loops will be a major operational problem in a highly heat integrated design.
U.S. Pat. No. 3,364,685 is a natural gas liquefaction process using a dual pressure, multistage, mixed refrigerant system in a closed loop. In contrast to U.S. Pat. No. 2,041,725, an intermediate pressure level is added for the mixed refrigerant. It appears that the thermodynamic inefficiency of U.S. Pat. No. 2,041,725 will be partially mitigated by the use of an intermediate pressure level in U.S. Pat. No. 3,364,685, although the additional complexity of distributing the vapor and liquid flows to an appropriate heating load will make the operational problems of U.S. Pat. No. 2,041,725 a greater challenge. The process of correcting imbalanced component ratios in the mixed refrigerant loop of U.S. Pat. No. 3,364,685 will be a confusing task even with careful inspection.
U.S. Pat. No. 4,274,849 is another natural gas liquefaction process with two closed refrigeration loops. The refrigeration loops are integrated so that one loop provides auto-refrigeration with flashed condensate and vapor and simultaneously, in the same pressure shell, indirectly chills the multi-component refrigerant of the second loop. Only the refrigerant of the second loop indirectly chills the natural gas. The second refrigeration loop is further integrated in the following options to chill the process stream. Partially or wholly condensed multi-component refrigerant provides auto-refrigeration with flashed condensate and/or vapor and simultaneously, in the same pressure shell, indirectly chills the process stream. The highly specific heat integration of the process is well understood to involve careful choice of component ratios in both mixed refrigerant loops, natural gas composition, relative pressure levels of the natural gas and the two mixed refrigerant loops. It is not an easy or clear choice to depart from the operation of the system as described in the working examples. Each of many equipment and process design decisions must be balanced against their influence on other equipment and process design decisions to make the system ultimately useful instead of so costly that it is effectively useless.
U.S. Pat. Nos. 4,504,296, 4,525,185 and 4,545,795 are consistent with the above statement in showing the large number of variations that might be made in natural gas liquefaction with two closed refrigeration loops using multi-component refrigerants. Partial condensation and separation of refrigerant streams in auto-refrigeration and in process chilling is not clearly preferred over using the multi-refrigerant streams without such separation. In fact, at the point that the processes require the coldest temperatures for auto-refrigeration or process chilling, most of the processes prefer a partial condensation and separation step to get the benefit of using a vapor stream reduced in heavy components to obtain a lower temperature refrigerant.
U.S. Pat. No. 4,720,293 uses a single closed mixed refrigerant loop for an initial chilling step for a demethanizer in ethylene recovery from cracked gas. The mixed refrigerant is ethane, propane and butane in the molar percentages of 48.3, 18.3 and 33.4. In order to achieve the objects of the patented concept, the ethane product and hydrogen must be combined to provide chilling for the initial chilling step. Such a result has little use in a commercial plant that must recycle ethane to the pyrolysis furnaces.
U.S. Pat. Nos. 4,900,347 and 5,035,732 describe the use of dephlegmators as an integral step in ethylene recovery from cracked gases. The patents share a consistent teaching of the use of single component refrigeration for dephlegmators, either in ethylene and propylene refrigeration loops or in the use of ethane recycle for refrigeration. U.S. Pat. No. 5,377,490 describes the use of a single, open, mixed refrigerant loop to chill two separate process condensation steps. A first chilling step is chilled by a part of mixed refrigerant in the open loop. The chilling duty supplied by the mixed refrigerant is 40% or less. A second condensation step uses a combined stream to chill a dephlegmator. The combined stream comprises the mixed refrigerant from the first chilling step and the net condensate from the dephlegmator. This combined stream is the only chilling stream used in the dephlegmator. The cracked gas is chilled to between -20 to -90 degrees F. before entering the dephlegmator and is the only chilling load in the dephlegmator.
High level ethylene recovery from cracked and other gases is an effort well worth making. The cost of lost ethylene in even very small amounts of ethylene is quite high for a process in continuous operation. However, the components associated with ethylene in cracked gas make such recovery costly in utilities and equipment for cryogenic separation. In addition, cracked gas does not have fixed relative amounts of its several components. Ethane derived cracked gas is very different from propane or naphtha derived cracked gas. Hydrogen, methane, ethylene and ethane vary widely in relation to each other depending on the source of the cracked gas.
Ethylene-propylene cascade refrigeration systems provide the predominant amount of refrigeration required in the ethylene plant. Most of the propylene (high level) refrigeration is utilized at several pressure/temperature levels in the initial feed precooling and fractionation sections of the plant, to cool the feed gas from ambient temperature to about -35.degree. F. and to condense the ethylene refrigerant at about -30.degree. F. Similarly, the ethylene (low level) refrigeration is utilized at several pressure/temperature levels in the cryogenic section of the plant to cool the feed from -35.degree. F. to about -145.degree. F. in order to condense the bulk of the ethylene in the form of liquid feeds to one or more demethanizer columns, and is used in at least one of the demethanizer column overhead condenser(s) at about -100.degree. F. to -145.degree. F. to provide reflux to the column(s). Ethylene is normally not used to provide refrigeration below -150.degree. F. since that would result in sub-atmospheric pressure at the suction of the ethylene compressor. Refrigeration below -150.degree. F. to condense the remaining ethylene from the feed gas is provided primarily by work expansion of hydrogen and methane-containing light gas streams and/or by vaporization of methane refrigerant which has been condensed by ethylene refrigerant. The work expanded gases are normally used as fuel and consist primarily of the overhead vapor from the demethanizer column, mostly methane, and any uncondensed feed gas, mostly hydrogen and methane, which is not processed in a hydrogen recovery section of the ethylene plant or ethylene recovery process. Refrigeration also may be recovered from one or more of the hydrogen-rich and methane-rich streams produced in a hydrogen recovery section.
Cooling and condensation of the feed gas can be accomplished with very high efficiency by dephlegmation in a dephlegmator, which is a rectifying heat exchanger which partially condenses and rectifies the feed gas. Typically a dephlegmator yields a degree of separation equivalent to multiple separation stages, typically 5 to 15 stages. The operation of the dephlegmator is different in kind from other separation devices. The continuous removal of heat from the bottom inlet to the top outlet increases the temperature level at which heat is removed from the rectification stages. The use of single component refrigerants, which provide refrigeration at only a single temperature, are inconsistent with obtaining the continuous heat transfer and separation benefits of dephlegmators.
Alternatively, cooling and condensation of the feed gas is accomplished in a conventional condenser, defined herein as a partial condenser, in which a feed gas is partially condensed to yield a vapor-liquid mixture which is separated into vapor and liquid streams in a simple separator vessel. A single stage of separation is realized in a partial condenser. The advantage to be gained by supplying 5-15 stages of separation in a dephlegmator over an equivalent number or partial condensers and drums is quite clear.
"Mixed Refrigerant Cascade Cycles" (Kinard, G. E. et al, Chemical Engineering Progress, January 1973, Vol. 69, No. 1, pp. 56-61) is an article describing the concept of cascaded, mixed refrigerant loops with reference to LNG. The article points out that significant entropy reduction in the LNG condensation can be obtained by using mixed refrigerants in a process with a single or multiple phase separation in a single mixed refrigerant loop. On page 58, the authors state "Larger cooling curve temperature differences give more irreversible cycles and cause a larger increase in entropy." Although in general entropy reduction is a desirable goal, the complexity of the above prior art processes designed to achieve that goal point out the difficulty in making significant entropy reductions and efficiency improvements through the use of mixed refrigerant cycles.
"Thermodynamic Analysis of Ethylene Plant Distillation Columns" (Manley, D. B. et al, 1992 AIChE Spring National Meeting, Section #85--Expansion and Life Extension Techniques in Ethylene Plants, New Orleans, La., Mar. 31, 1992, pp 1-8) describes "lost work" for each stage of an ethylene plant demethanizer, with separate analysis of momentum, mass transfer and heat transfer. "Lost work" is the difference between actual work done by a specific process and the ideal work available from the operation of any reversible process. On page 7, the authors state that the method of distillation column analysis " . . . identifies where distributed distillation and parallel heat transfer/fractionation can be effectively used to improve thermodynamic efficiency and reduce equipment loadings." FIG. 6 shows a comparison of reduced lost work in an improved demethanizer section design. Although general indications by said method of sections in the cooling curve where efficiency improvements might be made, innovation by the skilled person is still generally required to propose process designs which will be effectively analyzed by such methods.